Cardiac pathologic findings were analyzed in 20 necropsied patients from a series of 26 patients with leukemia, aplastic anemia, immune deficiency disease or metastatic cancer who had been treated with bone marrow transplantation. Most cardiac alterations were similar to those which occur in patients with hematologic and neoplastic diseases who have not been treated with bone marrow transplantation. Other cardiac alterations were more specifically related to bone marrow transplantation. Six patients exhibited a distinctive interstitial reactive change characterized by the presence of a pleomorphic population of lymphoid, histiocytic and Anitschkow cells. This alteration may have been induced by abnormal immune mechanisms. Two patients developed fatal cardiac failure in the post-transplant period, and exhibited myocardial damage with histologic and ultrastructural features indicative of severe acute injury. Clinicopathologic analysis strongly suggested that the fatal cardiotoxicity in both patients resulted primarily from effects of high doses of cyclophosphamide (180 mg/kg and 270 mg/kg) which were administered as part of a newly developed regimen of combination chemotherapy-immunosuppression (B.A.C.T.).